The invention relates to an air/fuel ratio detection device and an air/fuel ratio control device which can be mounted on a carburetor type, internal combustion engine for use in a small-sized boat, a small-sized generator, a mower etc., an engine having a fuel injection valve for use in an automobile etc., and other internal combustion engines.
Conventionally, in an engine mounted on the mower, no complex combustion control as is used in an automobile engine is used. In such small-sized, simple structures the number of revolutions and output are adjusted simply by using a carburetor and a throttle valve. The throttle valve is opened and the quantity of supply air is increased so as to increase the output and the number of revolutions.
The small engine, e.g. for mowers, is air cooled, and, as such, is less efficiently cooled as compared with a water cooled engine. In an air cooled engine, the concentration of fuel relative to air is high compared with a stoichiometric air/fuel ratio, at which the concentration of air is chemically equivalent to that of fuel. As a result, exhaust gas contains excessive uncombusted hydrocarbon.
Today research attempts to cope with environmental problems, by decreasing toxic components in the exhaust gas discharged from small engines.
However, the small engine is lightweight and cannot be provided with a catalytic converter, as this enlarges the entire size of the engine. Additionaly, such systems as in cars cannot be adopted in which the engine is combusted with the stoichiometric air/fuel ratio and the increase in temperature of the engine is controlled by circulating cooling water.
On the other hand, recently research has resulted in purified exhaust gas discharged from a water cooled automobile engine by controlling the air/fuel ratio to be lean, such that pollutions are prevented from being discharged by the engine immediately after the engine is started. A so-called universal air/fuel heated exhaust gas oxgen sensor for detecting the air-fuel ratio all over the region is required for such control.
The universal air/fuel heated exhaust gas oxygen sensor and the associated control devices presently available have a complicated, expensive structure, and are not very satisfactory for use in control executed just after the engine is started.